Black Hole Polarimetry I: A Signature of Electromagnetic Energy Extraction

Abstract

In 1977, Blandford and Znajek showed that the electromagnetic field surrounding a rotating black hole can harvest its spin energy and use it to power a collimated astrophysical jet, such as the one launched from the center of the elliptical galaxy M87. Today, interferometric observations with the Event Horizon Telescope (EHT) are delivering high-resolution, event-horizon-scale, polarimetric images of the supermassive black hole M87* at the jet launching point. These polarimetric images offer an unprecedented window into the electromagnetic field structure around a black hole. In this paper, we show that a simple polarimetric observable -- the phase $\angle {\beta }_2$ of the second azimuthal Fourier mode of the linear polarization in a near-horizon image -- depends on the sign of the electromagnetic energy flux and therefore provides a direct probe of black hole energy extraction. In Boyer-Lindquist coordinates, the Poynting flux for axisymmetric electromagnetic fields is proportional to the product $B^{\varphi }{B}^r$. The phase $\angle {\beta }_2$ likewise depends on the ratio $B^{\varphi }/B^r$, thereby enabling an observer to experimentally determine the direction of electromagnetic energy flow in the near-horizon environment. Data from the 2017 EHT observations of M87* are consistent with electromagnetic energy outflow. Currently envisioned multi-frequency observations of M87* will achieve higher dynamic range and angular resolution, and hence deliver measurements of $\angle {\beta }_2$ closer to the event horizon as well as better constraints on Faraday rotation. Such observations will enable a definitive test for energy extraction from the black hole M87*.

arXiv comment35 pages, 5 figures. Published in ApJ

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Cited by in corpus (15)

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• Jet Archaeology and Forecasting: Image Variability and Magnetic Field Configuration
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